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Fish gills areorgans that allowfish tobreathe underwater. Most fishexchange gases like oxygen and carbon dioxide using gills on both sides of thepharynx (throat). Gills possesstissues resembling short threads, referred to asgill filaments orlamellae. Each filament contains acapillary network that provides a largesurface area for exchangingoxygen andcarbon dioxide. Other than respiration, these filaments have other functions including the exchange ofions, water, acids, andammonia.[1][2]
Fish respire by pulling oxygen-rich water through their mouths and pumping it over their gills. Within the gill filaments, capillary blood flows in the opposite direction to the water, causingcountercurrent exchange. The gills push the oxygen-poor water out through openings in the sides of the pharynx. Some fish, likesharks andlampreys, possess multiple gill openings, but the most common group of fish alive, thebony fish, have a single gill opening on each side. This opening is hidden beneath a protective bony cover called theoperculum.
Juvenilebichirs have external gills, a very primitive feature that they share with larvalamphibians.
Previously, theevolution of gills was thought to have occurred through two diverging lines: gills formed from theendoderm, as seen injawless fish species, or those form by theectoderm, as seen injawed fish. However, recent studies on gill formation of the little skate (Leucoraja erinacea) has shown potential evidence supporting the claim that gills from all current fish species have in fact evolved from acommon ancestor.[3]
Allbasal vertebrates (types of fish) breathe withgills. The gills are carried right behind the head, bordering the posterior margins of a series of openings from theesophagus to the exterior. Each gill is supported by a cartilaginous or bonygill arch.[4] The gills ofvertebrates typically develop in the walls of thepharynx, along a series ofgill slits opening to the exterior. Most species employ acounter-current exchange system to enhance the diffusion of substances in and out of the gill, with blood and water flowing in opposite directions to each other.
The gills are composed of comb-like filaments, thegill lamellae, which help increase their surface area for oxygen exchange.[5] When a fish breathes, it draws in a mouthful of water at regular intervals. Then it draws the sides of its throat together, forcing the water through the gill openings, so that it passes over the gills to the outside. Thebony fish have three pairs of arches,cartilaginous fish have five to seven pairs, while the primitivejawless fish have seven. The vertebrate ancestor no doubt had more arches, as some of theirchordate relatives have more than 50 pairs of gills.[6]
Gills usually consist of thin filaments oftissue, branches, or slender tuftedprocesses that have a highly folded surface to increasesurface area. The high surface area is crucial to thegas exchange of aquatic organisms as water contains only a small fraction of thedissolved oxygen thatair does. Acubic meter of air contains about 250grams of oxygen atSTP. The concentration of oxygen in water is lower than air and it diffuses more slowly. In a litre offreshwater the oxygen content is 8 cm3 per litre compared to 210 in the same volume of air.[7] Water is 777 times more dense than air and is 100 times more viscous.[7] Oxygen has a diffusion rate in air 10,000 times greater than in water.[7] The use of sac-like lungs to remove oxygen from water would not be efficient enough to sustain life.[7] Rather than using lungs "Gaseous exchange takes place across the surface of highly vascularised gills over which a one-way current of water is kept flowing by a specialised pumping mechanism. The density of the water prevents the gills from collapsing and lying on top of each other, which is what happens when a fish is taken out of water."[7]
Higher vertebrates do not develop gills, the gill arches form duringfetal development, and lay the basis of essential structures such asjaws, thethyroid gland, thelarynx, thecolumella (corresponding to thestapes inmammals) and in mammals themalleus and incus.[6] Fish gill slits may be the evolutionary ancestors of thetonsils,thymus gland, andEustachian tubes, as well as many other structures derived from the embryonicbranchial pouches.[8][9]
Inbony fish, the gills lie in a branchial chamber covered by a bonyoperculum (branchia is an Ancient Greek word for gills). The great majority of bony fish species have five pairs of gills, although a few have lost some over the course of evolution. The operculum can be important in adjusting the pressure of water inside of the pharynx to allow proper ventilation of the gills, so that bony fish do not have to rely on ram ventilation (and hence near constant motion) to breathe. Valves inside the mouth keep the water from escaping.[6]
Thegill arches of bony fish typically have noseptum, so that the gills alone project from the arch, supported by individual gill rays. Some species retaingill rakers. Though all but the most primitive bony fish lack a spiracle, thepseudobranch associated with it often remains, being located at the base of the operculum. This is, however, often greatly reduced, consisting of a small mass of cells without any remaining gill-like structure.[6]
Fish transfer oxygen from the sea water to their blood using a highly efficient mechanism calledcountercurrent exchange. Countercurrent exchange means the flow of water over the gills is in the opposite direction to the flow of blood through the capillaries in the lamellae. The effect of this is that the blood flowing in the capillaries always encounters water with a higher oxygen concentration, allowing diffusion to occur all the way along the lamellae. As a result the gills can extract over 80% of the oxygen available in the water.
Marineteleosts also use their gills to excrete osmolytes (e.g. Na⁺, Cl−). The gills' large surface area tends to create a problem for fish that seek to regulate theosmolarity of their internal fluids. Seawater contains more osmolytes than the fish's internal fluids, so marine fishes naturally lose water through their gills via osmosis. To regain the water, marine fishes drink large amounts ofsea water while simultaneously expending energy to excretesalt through theNa+/K+-ATPase ionocytes (formerly known as mitochondrion-rich cells andchloride cells).[10] Conversely, freshwater has less osmolytes than the fish's internal fluids. Therefore, freshwater fishes must utilize their gill ionocytes to attain ions from their environment to maintain optimal blood osmolarity.[6][10]
In some primitive bony fishes andamphibians, thelarvae bear external gills, branching off from the gill arches.[11] These are reduced in adulthood, their function taken over by the gills proper in fishes and bylungs in most amphibians. Some amphibians retain the external larval gills in adulthood, the complex internal gill system as seen in fish apparently being irrevocably lost very early in the evolution oftetrapods.[12]
Sharks andrays typically have five pairs ofgill slits that open directly to the outside of the body, though some more primitive sharks have six or seven pairs. Adjacent slits are separated by acartilaginous gill arch from which projects a long sheet-likeseptum, partly supported by a further piece of cartilage called the gill ray. The individuallamellae of the gills lie on either side of the septum. The base of the arch may also supportgill rakers, small projecting elements that help to filter food from the water.[6]
A smaller opening, thespiracle, lies in the back of the first gill slit. This bears a smallpseudobranch that resembles a gill in structure, but only receives blood already oxygenated by the true gills.[6] The spiracle is thought to behomologous to the ear opening in higher vertebrates.[13]
Most sharks rely on ram ventilation, forcing water into the mouth and over the gills by rapidly swimming forward. In slow-moving or bottom dwelling species, especially among skates and rays, the spiracle may be enlarged, and the fish breathes by sucking water through this opening, instead of through the mouth.[6]
Chimaeras differ from other cartilagenous fish, having lost both the spiracle and the fifth gill slit. The remaining slits are covered by anoperculum, developed from the septum of the gill arch in front of the first gill.[6]
The shared trait of breathing via gills in bony fish and cartilaginous fish is a famous example ofsymplesiomorphy. Bony fish are more closely related toterrestrial vertebrates, which evolved out of a clade of bony fishes that breathe through their skin or lungs, than they are to the sharks, rays, and the other cartilaginous fish. Their kind of gill respiration is shared by the "fishes" because it was present in their common ancestor and lost in the other living vertebrates. But based on this shared trait, we cannot infer that bony fish are more closely related to sharks and rays than they are to terrestrial vertebrates.[14]
Lampreys andhagfish do not have gill slits as such. Instead, the gills are contained in spherical pouches, with a circular opening to the outside. Like thegill slits of higher fish, each pouch contains two gills. In some cases, the openings may be fused together, effectively forming an operculum. Lampreys have seven pairs of pouches, while hagfishes may have six to fourteen, depending on the species. In the hagfish, the pouches connect with the pharynx internally. In adult lampreys, a separate respiratory tube develops beneath the pharynx proper, separating food and water from respiration by closing a valve at its anterior end.[6]
Some fish can at least partially respire without gills. In some speciescutaneous respiration accounts for 5 to 40 per cent of the total respiration, depending on temperature. Cutaneous respiration is more important in species that breathe air, such asmudskippers andreedfish, and in such species can account for nearly half the total respiration.[15]
Fish from multiple groups can live out of the water for extended time periods.
Air breathing fish can be divided intoobligate air breathers andfacultative air breathers. Obligate air breathers, such as theAfrican lungfish, are obligated to breathe air periodically or they suffocate. Facultative air breathers, such as the catfishHypostomus plecostomus, only breathe air if they need to and can otherwise rely on their gills for oxygen. Most air breathing fish are facultative air breathers that avoid the energetic cost of rising to the surface and the fitness cost of exposure to surface predators.[16]
Catfish of the familiesLoricariidae,Callichthyidae, andScoloplacidae absorb air through their digestive tracts.[16]
Fish gills are the preferredhabitat of manyectoparasites (parasites attached to the gill but living out of it); the most commons aremonogeneans and certain groups of parasiticcopepods, which can be extremely numerous.[17] Other ectoparasites found on gills areleeches and, in seawater, larvae ofgnathiidisopods.[18]Endoparasites (parasites living inside the gills) includeencysted adultdidymozoidtrematodes,[19] a fewtrichosomoididnematodes of the genusHuffmanela, includingHuffmanela ossicola which lives within the gill bone,[20] and theencysted parasiticturbellarianParavortex.[21] Variousprotists andMyxosporea are also parasitic on gills, where they formcysts.
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